A prior art planar heating device 10 adapted to be attached to the backside of a vehicle mirror for the purpose of defogging, defrosting and/or de-icing will be described with reference to FIGS. 1A and 1B. A pair of main electrodes 3, 4 in the form of a strip are printed on one side surface of a flexible electrically insulating sheet 2 such as polyethylene terephthalate along the upper and lower end edges thereof in opposing relation with each other. FIG. 1A is an illustration of the insulating sheet 2 as seen through the other side surface thereof, assuming that the sheet is transparent. Power terminal lead electrodes 5 and 6 are formed such that they extend from the corresponding main electrodes 3, 4 toward each other, terminating in power terminal connections 5a and 6a, respectively in opposing proximity with each other. Comblike sub-electrodes 7 and 8 are formed by printing so as to extend from the corresponding main electrodes 3, 4 and power terminal lead electrodes 5 and 6 into interdigitated relation. A film 9 of resistive material is formed on the one side surface of the insulating sheet 2 to cover the sub-electrodes 7 and 8 as shown in FIG. 1B.
A pair of power terminals 11, 12 are staked to the other side surface of the insulating sheet 2 at the respective power terminal connections 5a and 6a by means of eyelet pieces 13 to be connected with the power terminal connections 5a and 6a. In many instances, the planar heating device 10 is completed with a double faced adhesive tape 15 applied to the one side surface of the insulating sheet 2 having the film 9 of resistive material formed thereon. The adhesive tape 15 is bonded to a deposited metallic chrome film 16b on the backside of a glass layer 16a which cooperates with the chrome film to form a mirror.
The power terminal lead electrodes 5, 6 and the proximal end portions of the main electrodes 3, 4 adjoining to the power terminal lead electrodes which allow flow of a large amount of electric current are made wider to prevent burning. On the other hand, the main electrodes 3, 4 are tapered in width towards their distal ends as the current flow decreases.
The conventional planar heating device 10 is affixed to the mirror 16 by applying the heating device formed on the one side surface of the flexible insulating sheet 2 such as polyester terephthalate to the mirror 16 by means of the double faced adhesive tape 15. However, such a planar heating device had the disadvantage of low efficiency in heat energy utilization due to the long time required for the heat in the resistance film 9 to be conducted to the mirror 16, since the double faced adhesive tape 15 has a low thermal conductivity and yet has a relatively thick thickness of 0.1 to 0.2 mm.
In addition, bonding the heating device to the mirror 16 involves peeling off the release paper of the double faced adhesive tape 15 bonded to one side surface of the planar heating device 10 and then applying the latter with the aid of an application jig, resulting in requiring cumbersome and time-consuming operations to mount the heating device to the mirror, which was in turn impedimental to manufacturing inexpensive mirrors with heaters.
Moreover, due to being formed on the flexible insulating sheet 2 made of an organic material such as polyethylene terephthalate (PET), the conventional planar heating device required that the electrodes comprise silver electrically conductive film containing thermosetting resin such as epoxy, phenol, melamine resin and the like as binder. In order to obtain a satisfactory adhesive strength, it was required that such thermosetting resin which is an electrically insulating material be compounded with a weight ratio of at least about 30%, resulting in an increase in the resistivity. This led to the need for enlarging the width of the electrodes to provide a high tolerance for electric current, which correspondingly reduced the effective surface area of the heating device as well as undesirably increasing the material cost.